Aqueous cosmetic compositions often require thickeners to give them an aesthetically pleasing viscosity. Also for a cosmetic to be effective, it is desirable to be substantive to the skin. Thickeners provide this substantivity. Furthermore, even though the products may be effective to the consumer, ineffectiveness can be indicated if the system has watery consistency i.e. low viscosity. Products of watery consistency are also aesthetically displeasing to consumers with expectations of rich and creamy products.
Formulators of cosmetic compositions are increasingly recognising the numerous benefits that alpha-hydroxy acids have to offer. The reported benefits attainable from the use of alpha-hydroxy acids include a tighter skin-feel and emollient action, giving the skin a softer and smoother aspect and also a healthier and more even-looking complexion with a useful and visible reduction of facial lines.
Some cosmetic formulations are extremely difficult to thicken and even when initially thickened may have storage stability problems. Low pH systems such as cosmetic formulations containing alpha-hydroxy acids (AHA in the rest of the description) are particularly sensitive and difficult.
Currently, thickeners used for low pH systems are limited to Magnesium Aluminium Silicate (obtainable from R. T. Vanderbilt Company Inc. under the trade name Veegum), Xanthan gum (obtainable from Kelco International under the trade name Keltrol 1000), Hydroxyethylcellulose (obtainable from Aqualon BV under the trade name Natrosol 25OHHR) and Polyacrylamide.
Cosmetic compositions containing AHAs and above mentioned thickeners are for example disclosed in W095/03811 or W094/27574. These materials deliver a degree of rheological control in low pH formulations, but are not ideal. The problems with these materials include:
(i) Poor viscosity control of magnesium aluminium silicate at low pH. PA1 (ii) A stringy/tacky characteristic of the cellulose. PA1 (iii) A definite "gummy" feel on the skin during rub out of the xanthan and cellulose. PA1 (iv) A degree of unpredictability relating to the procedure for addition of the polyacrylamide to the formulation. PA1 i) from about 0.01 to 20% by weight, preferably 0.1 to 10%, most preferably 3 to 8%, of C.sub.1 -C.sub.25 alpha-hydroxy carboxylic acids and their salts and mixtures thereof; PA1 ii) from about 0.05 to 0.5% by weight, preferably 0.1 to 0.3% of xanthan gum; PA1 iii) from about 0.5 to 8% by weight, preferably 1 to 5%, most preferably 2 to 4%, of an amorphous silica; PA1 v) a cosmetically acceptable carrier. PA1 i) from about 0.01 to 20% by weight, preferably 0.1 to 10%, most preferably 3 to 8.0%, of C.sub.1 -C.sub.25 alpha-hydroxy carboxylic acids and their salts and mixtures thereof, PA1 ii) a cosmetically acceptable carrier, PA1 iii) less than 0.5% by weight, preferably less than 0.1% by weight, of magnesium aluminium silicate, this cosmetic composition having a viscosity range from about 2,000 Pa s.sup.- to 80,000 Pa s.sup.-1, preferably 4,000 Pa s.sup.-1 to 45,000 Pa s.sup.-1, most preferably 8,000 Pa s.sup.-1 to 30,000 Pa s.sup.-1.
There is therefore a need for a thickener which is easily dispersed in the water phase of the formulation; is unaffected by high processing temperatures (75.degree.-800.degree. C.); is unaffected by acidic pH (3 to 5) and more importantly yields a thixotropic formulation. In addition, the final cosmetic formulation must deliver a "rich and creamy" skin-feel, which is easily absorbed into the stratum corneum and leaves little or no after-feel.
It is therefore a goal of the present invention to develop a thickening system which is effective at low pH and also stabilises oil and water emulsions to prevent syneresis. It is also a goal of the present invention to develop cosmetic compositions, which are stable at high temperatures.
It is a further goal of the present invention to develop a cosmetic composition containing AHA which delivers a "rich and creamy" skin-feel, is easily absorbed into the stratum corneum and leaves little or no after-feel.
It has now been found that a system comprising an amorphous silica and xanthan gum can be particularly effective.
Definitions, Tests and Procedures
i. Nitrogen surface area--pore volume
Nitrogen surface area is determined by standard nitrogen adsorption methods of Brunauer, Emmett and Teller (BET) using a multi point method with an ASAP 2400 apparatus supplied by Micromeritics of the U.S.A.. The samples are outgassed under vacuum at 270.degree. C. for at least one hour before measurement. Surface area is calculated from the volume of nitrogen gas adsorbed at p/po 0.98. This apparatus also provides the pore size distribution from which it is possible to get the pore size (D.sub.10) for which 10% of the pores are below this pore size. In the same manner, it is possible to get the pore size for which 50% (D.sub.50) and 90% (D.sub.90) of the pores are below this pore size. Additionally the pore volume (cm.sup.3 /g) for a given range of pore size can be obtained from the desorption curve. It is therefore possible to obtain the percentage of the pore volume contained in pores having a diameter between 10 and 30 nanometers.
ii. Rheology Analysis
Creams and lotions are generally viscoelastic materials possessing a yield point and shear thinning behaviour. The yield value of a material is defined as the stress required to shear or deform the material. Hence, a dynamic stress rheometer was used to determine the linear viscoelastic region, which is a measure of the inherent strength of the structure of the emulsion.
The Theological properties were measured using a Dynamic Stress Rheometer supplied by Rheometrics Inc. Epsom, England. Parallel plates were used having a diameter of 40 mm and a gap set at 0.542 mm with an auto-calibrated tool inertia value of 367.5 g/cm.sup.2. A dynamic stress sweep test was used to determine the dynamic viscoelastic properties of the AHA compositions at 25.degree. C. The dynamic stress sweep test increased linearly from 0 to 500 Pa at a frequency of 0.1 rad s.sup.-1 and increments of 50. A 60 second delay was employed prior to test to ensure consistency of sample application.
A 3D graph of storage or elastic modulus (G'), loss or viscous modulus (G") and viscosity was plotted against shear stress, for all compositions to determine the products' flow characteristics.
The shear stress contribution by gravity is approximately 20 Pa (Bell, 1988), therefore products with yield points below this will flow readily by themselves and hence appear runny. A lotion or cream with a yield value above 20 Pa will flow more slowly, giving the impression of `body`. However a product which retains its elasticity (G') at high stresses can be perceived as stringy/too viscous.
iii. Weight mean particle size
The weight mean particle size is determined with the aid of a Malvern Mastersizer using 45 mm path length lens. This instrument, made by Malvern Instruments, Worcestershire uses the principle of Mie scattering, utilising a low power He/Ne laser. Before measurement the sample was dispersed ultrasonically in water for a period of 7 minutes to form an aqueous suspension. The Malvern Mastersizer measures the weight particle size distribution of the silica. The weight mean particle size (d.sub.50), the 10 percentile (d.sub.10) and the 90 percentile (d.sub.90) are easily obtained from the data generated by the instrument.
iv. CTAB surface area
This method determines the specific surface area of samples, exclusive of area contained in micropores too small to admit hexadecyltrimethyl ammonium bromide (cetyltrimethyl ammonium bromide, commonly referred to as CTAB) molecules.
The isotherm for adsorption of an aqueous solution of CTAB at a charged surface has a long horizontal plateau corresponding to a bilayer coverage of the substrate surface. Rapid equilibration is achieved by using mechanical agitation. Titration with sodium dodecyl sulphate solution is used to determine the unadsorbed CTAB after removal of the dispersed silica by centrifugation.
Into a 50 cm.sup.3 screw-cap jar weight between 0.10 and 0.25 g of silica, depending upon surface area to be determined. For high surface areas, which lead to low CTAB titrations, the lower weight is employed. Add 25 cm.sup.3 of 0.01 mol.dm.sup.-3 CTAB solution and bring the pH of the mixture to 9.0 with 0.1 mol.dm.sup.-3 NaOH solution. Stopper the jar and agitate for 1 hour in a water bath set at 25.degree. C. Settle the suspension centrifugally and transfer 5 cm.sup.3 of the supernatant into a 50 cm.sup.3 measuring cylinder. Add 10 cm.sup.3 of deionised water, 15 cm.sup.3 of chloroform, 10 cm.sup.3 of mixed indicator solution (dimidium bromide/disulphine blue obtainable from BDH Ltd, Poole, Dorset, England) and titrate with 0.005 mol.dm.sup.-3 sodium dodecyl sulphate solution, previously calibrated by a standard CTAB solution. The titration end point is that point at which the chloroform layer becomes pale-pink. Record the volume of sodium dodecyl sulphate to reach the end point as V.sub.2 cm.sup.3. Conduct a blank titration in a similar manner on 5 cm.sup.3 of the stock CTAB solution and record the volume of sodium dodecyl sulphate as V.sub.1 cm.sup.3.
Calculate the CTAB surface per gram of silica by the following equation in which the calculation is based on a molecular cross section of the bromide of 35 .ANG..sup.2 : ##EQU1## Where W=Weight of silica sample (in grams) 0.5 accounts for bi-layer formation.